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The Scientific Revolution

From Aristotle to Einstein

2View of the Universe 500 Years Ago

3Based on ancient speculations: “The School of the Athens” by Raphael (1510)

4Plato (c.428-347 BC), the idealist, points to sky and his World of Ideas (perfection)Aristotle (384-322 BC), the realist, points to the ground (imperfection)

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Aristotle’s View of the Universe: 55 crystalline spheres, celestial objects attached to spheres, spheres rotated at different velocities, the Earth was at the center.

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Aristotle’s Prime Mover

Earth is at the center

of the Universe

All motion in the

heavens is uniform circular motion

Celestial objects are made from

perfect material and

cannot change their properties (e.g., their

brightness).

Three Guiding Principles

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Problem: Ancient Model Could Not ExplainRetrograde Motion & Varying Brightness

Point of Reference

8Solution: Epicycles and Deferents

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Movement of the Planets: As the center of the epicycle moves around the deferent, the planet moves around the epicycle. The apparent path

against the background stars is indicated by the blue line

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12Further Refinements: to account for the detailed motion of the planets, in

some cases, epicycles were themselves placed on epicycles.

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14The Ptolemaic Universe : Ideas about uniform circular motion and epicycles were catalogued by Ptolemy in 150 A.D. in his book the Almagest.

Ptolemy

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Medieval Representation of Ptolemaic Universe

STABILITY Earth was at center. Mankind

important in God’s plan

COMFORTIndividual could

locate God. Soul’s destination would be above or below.

REASSURANCEDivine power would

triumph over corruption and

decay of earthly things and lift the soul to an afterlife

in heaven

16Philosopher-theologian Thomas Aquinas (1222-1274) rediscovered Aristotle and blended his ideas with medieval theology.

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Aristotle’s Prime Mover became the

God of Christian theology.

The outermost sphere became

the Christian heaven.

Earth at center represented the Christian God’s

concern for mankind.

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Religious Dogma: Ideas originating with pagan Greek philosophers were incorporated into the Catholic church and became dogma. To challenge this

view of the universe was to challenge, not only science, but theology.

19Finally, Polish astronomer Nicolas Copernicus (1473-1543) proposed a sun-centered solar system. Did he get into trouble? No, On the Revolutions of the Heavenly Bodies was published on his deathbed .

20Copernicus’ Universe

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Contemporary Representation of Copernicus’ Solar System

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The Heliocentric System offers a simple explanation for varying brightness and retrograde motion

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Since the orbits of the planets are not circles but ellipses, Copernicus could not explain all the details of planetary motion without epicycles

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A Danish nobleman, Tycho Brahe (1546-1601), made important contributions: an astronomical observatory, precise instruments, extensive data on Mars (Kepler used to prove that the orbit of Mars was not a circle but an ellipse).

26Uraniborg Astronomical Observatory

27Brahe’s Observations: Proposed a model intermediate between the

Ptolemaic and Copernican models with Earth at center (widely accepted)

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Brahe’s Solar System

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Johannes Kepler (1571-1630), a German who went to Prague to become Brahe's assistant, realized that the orbits of the planets were not circles but ellipses and developed three laws to describe the phenomenon he observed.

30Kepler’s First Law: The orbits of the planets are ellipses, with the Sun at one focus of the ellipse.

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Kepler’s Second Law: The line joining the planet to the Sun sweeps out equal areas in equal times as the planet travels around the ellipse

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Kepler’s Third Law: The ratio of the squares of the revolutionary periods for two planets is equal to the ratio of the cubes of their semi major axes

33HTML Version of Kepler’s Law

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Galileo Galilei (1564-1642) proved the Copernican theory with his telescope, challenged Aristotle's universe and its theological-philosophical worldview, and laid the foundations for dynamics (how objects move on the earth) and gravity.

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Sunspots

Galileo observed sunspots that moved, indicating that the Sun was rotating on an axis and that it was not made from a perfect, unchanging substance.

He observed four points of light that changed their positions around the planet Jupiter and concluded that they were moons circling the planet as it moved around its orbit.

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Galileo used his telescope to show that Venus went through a complete set of phases, just like the Moon. This observation confirmed the Copernican system and proved that the Ptolemaic system was incorrect.

37Galileo's challenge of the Church's authority got him into deep trouble with the Inquisition. Late in his life, he was forced to recant his Copernican views publicly.

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Sir Isaac Newton (1642-1727) Newton demonstrated that the motion of objects on the Earth could be described by three new Laws of Motion and the Universal Law of Gravitation.

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Newton’s First Law of Motion: An object in motion tends to stay in motion and an object at rest tends to stay at rest, unless the object is acted upon by an outside force

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Free Fall Air Resistance

Newton’s Second Law of Motion: The acceleration of an object is directly pro-portional to the net force acting upon it and inversely proportional to its mass.

41Newton’s Third Law of Motion: Every action has an equal and opposite reaction.

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When he observed an apple fall from a tree, Newton thought: The apple is accelerated as it moves from the tree toward the ground. There must be a force that acts on the apple to cause this acceleration. Let's call the force gravity. If the force of gravity reaches to the top of the highest tree, might it not reach even further to the orbit of the Moon. Then, the orbit of the Moon about the Earth could be a consequence of the gravitational force.

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64. Newton’s Excellent Idea

Now came Newton's truly brilliant insight: if the force of gravity reaches to the top of the highest tree, might it not reach even further to the orbit of the Moon. Then, the orbit of the Moon about the Earth could be a consequence of the gravitational force.

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Newton's theory of gravitation was soon accepted without question, and it remained unquestioned until the beginning of this century when Albert Einstein (1879-1955) shook the foundations of physics with the introduction of his Special Theory of Relativity (1905) General Theory of Relativity (1915).

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Einstein Explains the Equivalence of Energy and Matter

It followed from the special theory of relativity that mass and energy are both but different manifestations of the same thing -- a some-what unfamiliar conception for the average mind. Furthermore, the equation E is equal to m c-squared, in which energy is put equal to mass, multiplied by the square of the velocity of light, showed that very small amounts of mass may be converted into a very large amount of energy and vice versa. The mass and energy were in fact equivalent, according to the formula mentioned before. This was demonstrated by Cockcroft and Walton in 1932, experimentally.